The claims came after the Observer revealed last week that the private outsourcing giant is to be investigated by MPs when it was forced to disclose a secret internal report revealing evidence that it failed to properly investigate a claim of repeated sexual assaults by one of its staff against a female resident.

The whistleblower also claimed that another alleged case of sexual assault by a Serco member of staff occurred in August 2012, involving a particularly vulnerable detainee with profound psychological issues. It is understood she has since been deported.

The claims come from the first senior employee to have broken rank since the immigration detention centre – which is so tightly guarded that the Home Office recently banned the United Nations from entry – opened in 2001.

The whistleblower claims Yarl’s Wood is not fit for purpose and that he detected a culture of disbelief towards female detainees, claims which are rejected by Serco.

He said: “Officers would say openly: ‘They need to go back, they need to leave the country, they’re only coming here to use NHS resources.’ A common phrase was: ‘They’re only putting it on to block their removal.’ I’ve actually heard [senior staff] say: ‘These people are putting it on.’ It was endemic … even the senior management structures were saying this, it was a mindset.”

The whistleblower, who resigned from Serco last year after raising concerns, corroborated allegations from former detainees that some women felt they had to flirt with staff to obtain everyday essentials such as toiletries.

He also claimed that a Serco female officer had told him of a “blind spot” inside Yarl’s Wood, which was not covered by CCTV and was a well-known location for trysts and where an officer had previously made a woman detainee pregnant.

“She said: ‘This is a blind spot, this is where people come.’ They seemed to know where all the blind spots were,” said the whistleblower.

He also alleged that mental health concerns were not dealt with adequately and that too many women were being deported without sufficient assessment.

“The lack of engagement with mental health in relation to assessment and safeguards was very concerning. They weren’t doing assessments to rule out mental health, the ACDT [Assessment Care in Detention and Teamwork] documentation wasn’t getting filled out properly. God knows how many people they had deported without a proper assessment,” he told the Observer.

The whistleblower estimated that more than half of the detainees at any time – over 200 individuals – had either self-harmed or were at risk of self-harming. Serco, however, cites the latest chief inspector of prisons report, which found “good primary mental health provision” inside the centre.

A possible reason for the alleged failure to assess inmates was, he believed, to ensure women were deported faster. “It’s clear to me that they didn’t specifically want to identify the mental health needs of individuals because that would block the system”.

He described one instance where a colleague told him that a woman with severe kidney failure was earmarked for deportation because “she’s taken up the resources of the NHS and needs to be deported. She was a dying woman.”

The former Serco employee also alleged that guards could be hostile and intimidating to some of the detainees and that officers would often complain about understaffing which, he said, manifested itself in frustration. …

Among incidents that he witnessed were a woman who poured boiling water over herself and was left for hours in a state of shock and a woman who tied a ligature around her neck while she was apparently under observation.

Cristel Amiss of the Black Women’s Rape Action Project said the whistleblower’s claims chimed with reports they received from former detainees. She said: “The revelations of the abuse and mistreatment of vulnerable women have come about because of women’s courageous decision to speak publicly and the Observer‘s determination to pursue this issue.”

USA: Immigrant detention facilities are using their own detainees as cheap labor: here.